Bug: Incompatible uniqueness assumption causes crashes

[emchristiansen]

Code to reproduce for OwnedTape<_, _>:

let x = dev.tensor(1.0);
let y = x
  .clone()
  .put_tape(OwnedTape::<f32, Cpu>::default())
  + x.with_empty_tape();
// Crash is here.
y.backward();

Code to reproduce for Arc<Mutex<OwnedTape<_, _>>>:

let dev = Cpu::default();
let x = dev
  .tensor(1.0)
  .put_tape(Arc::<
    Mutex<OwnedTape<f32, Cpu>>,
  >::default(
  ));
let y = x.clone() + x;
// Crash is here.
y.backward();

The code crashes with this error:

Message:  assertion failed: `(left != right)`
  left: `UniqueId(0)`,
 right: `UniqueId(0)`
Location: dfdx/src/tensor/gradients.rs:143

The crash is caused by the assert here, which appear to be related to a requirement that operands represent unique tensors.

[coreylowman]

Hmm yeah thanks for raising this. As a workaround, you can probably just use 2 * x instead of x + x in the meantime.